Glossary Vv

VACANCY – Unoccupied sites within a crystal that are usually occupied by an atom. A vacancy may move when a neighboring atom or ion moves to occupy the vacant site (the vacancy shift in the opposite direction to the site that was the source of the atom). The surrounding bonded crystal structure ensures that the neighboring atoms do not collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, because they can better form bonds with atoms in the other directions.

VACUUM – Space entirely devoid of matter (called also, by way of distinction, absolute vacuum). In a more general sense, a space, as the interior of a closed vessel, which has been evacuated to a high degree by a pump or other artificial means.

VALENCE – Charge on an ion reflecting gained or lost electrons. For example, an iron atom that has lost two electron, Fe2+, is described as having a valence of 2. Typical valences in geological materials are shown below.

VALENCE BAND – Outermost energy band that contains electrons when a solid is in the ground state. An intrinsicsemiconductor (or insulator) in its ground state has a completely filled valence band, while the conduction band above the valence band is completely empty.

ALENCE ELECTRON – Electron in the valence shell of an atom, or the valence band of a solid. Valence electrons are more active than non-valence electrons, so valence electrons are responsible for most of an atom's or solid's electrical and chemical properties.

VALENCE SHELL– Outermost shell of electrons in an atom in its ground state.

VARIABLE STAR – Star whose luminosity varies over time. Broadly speaking, variable stars are of two types: (1) stars that are intrinsically variable, that is, their luminosity actually changes, for example because the star periodically swells and shrinks; (2) eclipsing and rotating variables, where the apparent changes in brightness are a perspective effect. The first 334 variable stars discovered in a constellation are given a one or two letter code such as R Scuti or UV Ceti. Other variable stars are designated V335, V336, etc. For example, Proxima Centauri is known to variable star astronomers as V645 Centauri.

VELOCITY – Vector measuring the time rate of change of displacement. It is specified by magnitude (the speed) and direction. In general, the velocity is a function of time:

where is r(t) the three-dimensional vector displacement, also written as a function of time. The velocity is expressed in units of [distance/time], often m/s or km/h, with the direction of motion specified as well.

VELOCITY DISPERSION – Random motions (orbits) of stars which support a self-gravitating body against collapse. Motions can be either ordered or random. The most prevalent form of ordered motion is rotation, in which the majority of stars orbit in the same direction. A galaxy with strong rotation is “rotation supported.” In contract, elliptical galaxies and the stellar halos and bulges of spiral galaxies show little or no rotation (as many stars orbiting in one direction as in another). Such a galaxy is “velocity dispersion” (or pressure) supported. Rotation can be measured by determining the galactic redshift at a number of positions across the galaxy. Measurement of velocity dispersion requires a measure of the velocity broadening of spectral lines.

VERTICAL CIRCLE – Arc of a great circle drawn from the zenith through a star and perpendicular to the horizon.

VESTA - Third largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the interior was at one time molten, heated by short-lived radio-isotopes. A deep impact crater 456 km wide (on a world itself not much over 500 km across!), visible to the Hubble Space Telescope, exposes 4 Vesta’s mantle.

The theory is that, during the impact event, large chunks of matter were ejected from 4 Vesta to form smaller asteroids of similar composition – so-called Vestoids. Some of these bits of 4 Vesta subsequently entered near-Earth orbits, and are thought to be the sources of the howardites, eucrites, and diogenites.

Vesta is located in a part of the main asteroid belt that makes it almost impossible for it to send meteorites to us. So there are probably intermediate asteroids, which were once part of Vesta, located in more favorable orbits that provide delivery. This theory has been bolstered by the discovery that the asteroid 1929 Kollaa, based on its reflectance spectrum, was once a part of Vesta, and, moreover, that it moves in an orbit from which meteorites could much more easily be launched Earthward.

VIRAL THEOREM – Theorem that relates the total kinetic energy of a self-gravitating body due to the motions of its constituent parts (T) to the gravitational potential energy (U) of the body.

By re-arranging and making some simple assumptions about T and U for galaxies, this becomes:

where M is the total mass of the galaxy, v is the mean velocity (combining the rotation and velocity dispersion) of stars in the galaxy, G is Newton's gravitational constant, and R is the effective radius (size) of the galaxy. This equation relates two observable properties of galaxies (velocity dispersion and effective radius) to a fundamental, but unobservable, property - the mass of the galaxy. The comparison of mass estimates based on the virial theorem to estimates based on the luminosities provides one method of estimating the amount of dark matter in a galaxy or clusters of galaxies.

VIRGO CLUSTER – Largest and nearest galaxy cluster to the Local Group. The Local Group is often considered to lie on the edge of the Virgo cluster. The Virgo Cluster is approximately 16 Mpc distant and spans 8° on the sky. Its mass is ~1 x 1014 Msun, and it contains ~ 2,000 galaxies, with many more spiral galaxies than typical for a cluster of this size. There are three clearly identifiable sub-clusters (centered on M87, M86, and M49) and has an irregular X-ray halo (image), suggesting that the cluster is still forming.

The mass of the Virgo cluster is so large that the motions of most galaxies in its neighborhood are influenced by it and drawn towards it. Although the Local Group is current receding from the cluster, the mass of the Virgo cluster is so high that it is expected that the Local Group will eventually slow down and reverse direction, ultimately joining the cluster.

VOID – Huge region of space that is unusually empty of galaxies. Voids are not entirely empty, but are underdense and contain far fewer bright galaxies than average.

VOLATILE – Substances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.).

VOLATILIZE – Conversion of liquids into gases, usually by heating them.

VOLCANIC - Igneous rock that forms from cooling magma on the surface of a planet or asteroid.

VOLTAGE (V) – Change in energy per unit of electric charge, measured in the SI unit of volt (V). For example, a 1.0 Volt battery increases the energy of each Coulomb of charge by one Joule. Voltage is also called “potential” or “potential difference.”